How to Make a Single Turn Worm Gear with a CNC

This page will show you how to make a single-turn worm gear with a CNC. Even if you don’t have a CNC, this tutorial might still be useful, so don’t leave yet!

Notice how I said “single” turn worm gear. Although it might be possible to make a worm gear that can rotate more than 360 degrees, doing so would be difficult as the circumference would have to be exactly perfect. If it’s not, after just one rotation the threads won’t line up correctly and the worm gear will slowly be ruined.

It’s hard to know exactly what the gear ratio is because of the way the worm gear is made, but I’ve figured it to be about 155.5:1.

There are, of course, improvements that can be made to the design, depending on how it is going to be used. In particular, this should work pretty well for high resolution motion because of the high gear ratio. If you wanted to lift a heavy weight with it though, you would need to make some changes to the way the plastic circle attaches to the center threaded rod to keep it from loosening the nuts.

Download

Here are the CamBam files for this project. Make sure to double check everything, and, if something is wrong, please let me know.

Parts Laid Out

Below you can see a couple of pictures of the worm gear parts. Most everything is made out of OSB board. OSB isn’t really that pretty, but it’s all that I had on hand. I’m also trying to get away from that MDF stuff because the dust from it is downright gross, so I figured that I’d give OSB a try and see how it works out.

Here is the one plastic part used for this project. It a circle with a 3″ diameter that will later have the threads from a threaded rod imprinted around the outside of it.

To make this worm gear, you will also need

four 7/8″ roller skate bearings

two 5/16″ threaded rods: one at least 4 1/2″ long and another at least 7″

eight 5/16″ nuts

four 1/4″ threaded rods: one at least 4″ long, two more about 5″ long, and one about 6″ long.

sixteen 1/4″ nuts

(This list is only really an approximation, and many of the items can be substituted with others.)

Strange Mistake

You can see in the picture below that the parts don’t fit together quite like they should. This was a software problem that I was able to fix easily just by increasing the acceleration in the machine controller. The gaps won’t hurt anything and might actually come in handy if I want to make adjustments, so I’m just going to leave it the way it is. I can always fill them with glue later.

Cutting the Threads into the Worm Gear

In the picture below, you can see how most everything is already. Between the pictures and the Sketchup model at the bottom of this page, you shouldn’t have any trouble figuring out what goes where, so I’m not going to go through those steps.

It’s rather difficult to show you how to cut the threads using only pictures, but I’ll do the that best I can.

The first step for making them is to cut a groove in the middle of the plastic circle. To do this, attach a power drill to the shaft of the plastic circle. While the power drill spins it, hold the threaded rod firmly against the circle like you see in the picture below.

If you look closely at the picture below, you can see the groove beginning to form. Make sure not to go too deep.

In the picture below this text, I have stopped just short of the point where it would be easy to push the threaded rod all the way through both roller skate bearings. In order to do so, I am bending it over the plastic circle slightly with a wrench so that I can get it into the hole.

It is kind of hard to see what going on, but the main thing here is to make sure that the threaded rod is pushing against the plastic circle with a little bit of pressure once you get it all the way through both roller skate bearings.

If there is too much pressure, you need to cut the groove deeper. If there is not enough pressure, well then you cut the groove too deep and will probably have to start over with a new plastic circle.

It’s about finished now. The only thing left is to cut the threads into the plastic. In the pictures both above and below this text, you can see that I made four short lines with a green marker on the plastic circle. These are there to mark the point where the threads need to stop. (Remember how I said it’s just a single turn worm gear.)

To actually make the threads, attach a power drill to the threaded rod and simply run it one direction until you reach the marks and then reverse in the opposite direction until you reach those same marks again. Do this a few times to get a good, clean indentation. Note: Make sure that the friction doesn’t create too much heat at this point because you might end up melting the plastic.

Here is a picture of the threads after being cut into the plastic nut. This is the opposite side of the worm gear from the above picture. I took out one of the bolts that holds the worm gear together so you can see it better. Otherwise, it would be sitting directly in the way.

Pictures

Here are some picture of it finished.

Sketchup Model

Here is the Sketchup model of this worm gear. If you’re making your own, it should hopefully show you where to put all of the bolts and things. There are a few holes that don’t have anything in them. These were purposely left empty. I also didn’t bother to draw the nuts on the bolts because it should be fairly obvious where they go.

This worm gear has been working well so far. It will be interesting to see how it holds up over the long term, but I think it should do OK.

If you have any questions about this project, you can ask me about it on the forum.